I.1 Methods of Fractionation and Sorting
Among analytical separation techniques, methods of flow cytometry have been used in sorting of particles (including cells) within a given heterogeneous population. These methods, however, require a dedicated instrument capable of selecting and physically separating from the population those particles which satisfy a given selection criterion. For example, the objective of cell sorting by flow cytometry usually is the selection of those cells within a population displaying a characteristic fluorescence signal.
I.2 Methods of Particle Analysis
Particle analysis represents a standard procedure of the analytical chemical repertoire that is used to determine physico-chemical and compositional properties [Hunter, “Introduction to Modern Colloid Science”, Oxford University Press, Oxford, UK, 1993]. The extensive repertoire of techniques reflects the ubiquitous use of particles of a wide range of sizes, shapes, composition and chemical reactivity in many scientific and industrial applications ranging from medical diagnostics to cosmetics. Characterization of particles is useful in guiding and optimizing production as well as chemical modification particularly of surface properties that determine the stability of particle suspensions and the interaction of particles with molecules in the surrounding medium.
Techniques of the standard repertoire may be grouped as follows: first, sedimentation and centrifugation methods, electroacoustics, light scattering, hydrodynamic methods and dielectric spectroscopy; second, electrical pulse counting, flow cytometry and electrophoretic zeta potential measurements; third, specialized methods including dielectrophoresis (DEP), including its combination with field-flow fractionation methodology.
Methods in the first group apply to bulk suspensions and therefore deliver an average of the measured quantities for a large number of particles. While desirable in situations in which large industrial batches are to be characterized, bulk measurements generally do not lend themselves to miniaturization and are not well suited to the characterization of small numbers of particles, particularly when particle-to-particle variations are of interest or when multiple particle parameters are to be determined simultaneously.
Methods in the second group use a “single-file” serial format of measurement. While these methods determine the properties of individual particles, their implementation, in order to attain sufficiently high processing rates, requires high flow rates of a carrier gas or fluid and high-speed data recording and read-out electronics, generally rendering the equipment complex and expensive. This is so especially when multiple parameters are to be determined for each particle. For example, multi-color detection in flow cytometers can require the use of multiple lasers and multiple photomultiplier tubes. The determination of the zeta potential along with particle size in state-of-the-art equipment requires the measurement of electrophoretic mobility of particles traveling along a narrow capillary in response to a DC voltage applied along the channel as well as the application of light scattering with associated light source and read-out, as in the ZetaSizer (Malvern Instruments, Southborough, Mass.). Miniaturization, as in the case of flow cytometry [Fu, AX, Spence, C., Scherer, A., Arnold, F. H. and Quake, S. R. A Microfabricated Fluorescent-Activated Cell Sorter, Nature Biotechnology, Vol. 17, November 1999, 1109–1111], retains the serial mode of processing and thus encounters the constraint of limited throughput.
Methods in the third group include various dielectrophoretic techniques to characterize, classify and fractionate low conductivity particle suspensions [Becker, F. F., Gascoyne, P. R. C., Huang, Y. and Wang, X-B, Method and Apparatus for Fractionation Using Generalized Dielectrophoresis and Field Flow Fractionation, U.S. Pat. No. 5,888,370, Mar. 30, 1999; Rousselet, J., Markx, G. H. and Pethig, R., Separation of Erythrocytes and Latex Beads by Dielectrophoretic Levitation and Hyperlayer Field-Flow Fractionation, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 140 (1998) 209–216; Pethig, R., Markx, G. H., Apparatus for Separating by Dielectrophoresis, U.S. Pat. No. 5,814,200, Sep. 29, 1998; Pohl, H. A., Continuous Dielectrophoretic Cell Classification Method, U.S. Pat. No. 4,326,934, Apr. 27, 1982; Parton, A., Huang, Y., Wang, X-B., Pethig, R., MacGregor, A. R., and Pollard-Knight, D. V., Methods of Analysis/Separation, U.S. Pat. No. 5,653,859, Aug. 5, 1997; Crane, S., Dielectrophoretic Cell Stream Sorter, U.S. Pat. No. 5,489,506, Feb. 6, 1996; Benecke, W., Wagner, B., Hagedorn, R., Fuhr, G., Muller, T., Method of Continuously Separating Mixtures of Microscopic Dielectric Particles and Apparatus for carrying through this method, U.S. Pat. No. 5,454,472, Oct. 3, 1995]. All publications which are cited throughout the application are hereby incorporated by reference in their entirety.
Coupled with a suitable imaging system, techniques within this group can provide single particle information. The particle classification is achieved by exposing the particle mixture to a non-uniform AC electric field which is generated by applying an AC voltage to multiple sets of planar patterned microelectrodes. Such a scheme in general requires a complex electrode signaling/addressing scheme. Also, a continuous mode of operation requires, in the simplest case, coupling to an external flow or the use of field flow fractionation. Simple dielectrophoretic setups do not generally allow for quantitative evaluation for the surface potential or surface conductivity of particles. While these quantities maybe determined from electrorotation spectra of single particles, this approach requires a complex and time-consuming set-up to confine a single particle at the origin of a rotating electric field whose frequency is scanned while recording the rotational motion of the confined particle. Aside from the complexity of the equipment required to implement the requisite experimental configuration, the low throughput of the method presents a serious disadvantage when averages of multiple particles are desired.